Inward-rectifier potassium channel: Difference between revisions

'''Inward-rectifier potassium channels''' ('''K_ir''', '''IRK''') are a specific [[Lipid-gated_ion_channels|lipid-gated]] subset of [[potassium channel]]s. To date, seven subfamilies have been identified in various mammalian cell types,<ref name="Kubo">{{cite journal | vauthors = Kubo Y, Adelman JP, Clapham DE, Jan LY, Karschin A, Kurachi Y, Lazdunski M, Nichols CG, Seino S, Vandenberg CA | display-authors = 6 | title = International Union of Pharmacology. LIV. Nomenclature and Molecular Relationships of Inwardly Rectifying Potassium Channels | journal = Pharmacological Reviews | volume = 57 | issue = 4 | pages = 509–26 | date = December 2005 | pmid = 16382105 | doi = 10.1124/pr.57.4.11 | s2cid = 11588492 | authorlink8 = Colin Nichols }}</ref> plants,<ref name="pmid8582318">{{cite journal | vauthors = Hedrich R, Moran O, Conti F, Busch H, Becker D, Gambale F, Dreyer I, Küch A, Neuwinger K, Palme K | display-authors = 6 | title = Inward rectifier potassium channels in plants differ from their animal counterparts in response to voltage and channel modulators | journal = European Biophysics Journal | volume = 24 | issue = 2 | pages = 107–15 | year = 1995 | pmid = 8582318 | doi = 10.1007/BF00211406 | s2cid = 12718513 }}</ref> and bacteria.<ref name="tcdb.org">{{Cite web|url = http://www.tcdb.org/search/result.php?tc=1.A.2|title = 1.A.2 Inward Rectifier K Channel (IRK-C) Family|website = TCDB|access-date = 2016-04-09}}</ref> They are activated by phosphatidylinositol 4,5-bisphosphate ([[Phosphatidylinositol 4,5-bisphosphate|PIP₂]]). The malfunction of the channels has been implicated in several diseases.<ref>{{cite journal | vauthors = Hansen SB | title = Lipid agonism: The PIP2 paradigm of ligand-gated ion channels | journal = Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids | volume = 1851 | issue = 5 | pages = 620–8 | date = May 2015 | pmid = 25633344 | pmc = 4540326 | doi = 10.1016/j.bbalip.2015.01.011 }}</ref><ref name="Abraham">{{cite journal | vauthors = Abraham MR, Jahangir A, Alekseev AE, Terzic A | title = Channelopathies of inwardly rectifying potassium channels | journal = FASEB Journal | volume = 13 | issue = 14 | pages = 1901–10 | date = November 1999 | pmid = 10544173 | doi = 10.1096/fasebj.13.14.1901 | doi-access = free | s2cid = 22205168 }}</ref> IRK channels possess a pore domain, homologous to that of [[voltage-gated ion channel]]s, and flanking [[transmembrane domain|transmembrane segments]] (TMSs). They may exist in the membrane as [[Homo-oligomer|homo-]] or [[Hetero-oligomers|heterooligomers]] and each monomer possesses between 2 and 4 TMSs. In terms of function, these proteins transport [[potassium|potassium (K⁺)]], with a greater tendency for K⁺ uptake than K⁺ export.<ref name="tcdb.org"/> The process of inward-rectification was discovered by [[Denis Noble]] in cardiac muscle cells in 1960s<ref>{{Cite journal |last=Noble |first=Denis |date=December 1965 |title=Electrical properties of cardiac muscle attributable to inward going (anomalous) rectification |url=https://onlinelibrary.wiley.com/doi/10.1002/jcp.1030660520 |journal=Journal of Cellular and Comparative Physiology |language=en |volume=66 |issue=S2 |pages=127–135 |doi=10.1002/jcp.1030660520 |issn=0095-9898}}</ref> and by [[Richard Adrian, 2nd Baron Adrian|Richard Adrian]] and [[Alan Lloyd Hodgkin|Alan Hodgkin]] in 1970 in skeletal muscle cells.<ref>{{cite journal | vauthors = Adrian RH, Chandler WK, Hodgkin AL | title = Slow changes in potassium permeability in skeletal muscle | journal = The Journal of Physiology | volume = 208 | issue = 3 | pages = 645–68 | date = July 1970 | pmid = 5499788 | pmc = 1348790 | doi = 10.1113/jphysiol.1970.sp009140 }}</ref>

==Role of K_ir channels==

K_ir channels are found in multiple cell types, including [[macrophages]], [[cardiac]] and [[kidney]] cells, [[leukocytes]], [[neurons]], and [[endothelial cells]]. By mediating a small [[depolarization|depolarizing]] K⁺ current at negative membrane potentials, they help establish resting membrane potential, and in the case of the [[G protein-coupled inwardly-rectifying potassium channel|K_ir3]] group, they help mediate inhibitory [[neurotransmitter]] responses, but their roles in cellular physiology vary across cell types:

==Classification of K_ir channels==

There are seven subfamilies of K_ir channels, denoted as K_ir1 -– K_ir7.<ref name="Kubo"/> Each subfamily has multiple members (i.e. K_ir2.1, K_ir2.2, K_ir2.3, etc.) that have nearly identical amino acid sequences across known mammalian species.

K_ir channels are formed from as [[homotetrameric]] membrane proteins. Each of the four identical protein subunits is composed of two membrane-spanning [[alpha helix|alpha helices]] (M1 and M2). Heterotetramers can form between members of the same subfamily (i.e. K_ir2.1 and K_ir2.3) when the channels are overexpressed.